Linear motor

ABSTRACT

An electric linear motor for driving a reciprocating load includes a stator, a coil, an armature and a commutation circuit. The stator has a magnetically permeable core with an air gap. The coil is wound around a portion of the stator and energized with a non-constant voltage for producing a non-constant magnetic flux in the stator and the air gap. The armature supports a permanent magnet having a substantial portion located in the air gap. The interaction of the magnetic field of the magnet and the non-constant flux in the air gap produces a force on the armature. The armature is connected to the load and reciprocates with respect to the stator. The circuit controls the coil such that at least one end of the magnet passes outside the region of substantially uniform flux density within the air gap during a portion of the motion of the armature.

This application is a divisional application of Ser. No. 10/018,323,filed on Dec. 11, 2001 and assigned a filing date under 35 U.S.C. §371of Apr. 5, 2002, which is the United States National Stage patentapplication of PCT/NZ00/00105 which has an international filing date ofJun. 21, 2000 and which was published in English on Dec. 28, 2000 underInternational Publication No. WO 00/79671.

TECHNICAL FIELD

This invention relates to a compact linear motor including free pistoncompressors (also called vibrating and linear compressors) for vapourcompression systems and in particular a control system to preventfailure or damage due to unwanted changes of compression level caused bychanges to ambient temperature or operating conditions.

BACKGROUND ART

Compressors, for example refrigerator compressors, are conventionallydriven by rotary electric motors. However, even in their most efficientform, there are significant losses associated with the crank system thatconverts rotary motion to linear reciprocating motion. Alternatively arotary compressor which does not require a crank can be used but againthere are high centripetal loads, leading to significant frictionallosses. A Linear compressor driven by a linear motor would not havethese losses, and can be designed with a bearing load low enough toallow the use of aerostatic gas bearings as disclosed in U.S. Pat. No.5,525,845.

Linear reciprocating motors obviate the need for crank mechanisms whichcharacterise compressors powered by rotating electric motors and whichproduce high side forces requiring oil lubrication. Such a motor isdescribed in U.S. Pat. No. 4,602,174. U.S. Pat. No. 4,602,174 disclosesa linear motor design that is extremely efficient in terms of bothreciprocating mass and electrical efficiency. This design has been usedvery successfully in motors and alternators that utilise the Stirlingcycle. It has also been used as the motor for linear compressors.However, in the case of compressors designed for household refrigeratorsthe design in U.S. Pat. No. 4,602,174 is somewhat larger and more costlythan is desirable for this market.

The piston of a free piston compressor oscillates in conjunction with aspring as a resonant system and there are no inherent limits to theamplitude of oscillation except for collision with a stationary part,typically part of the cylinder head assembly. The piston will take up anaverage position and amplitude that depend on gas forces and inputelectrical power. Therefore for any given input electrical power, aseither evaporating or condensing pressure reduces, the amplitude ofoscillation increases until collision occurs. It is therefore necessaryto limit the power as a function of these pressures.

It is desirable for maximum efficiency to operate free pistonrefrigeration compressors at the natural frequency of the mechanicalsystem. This frequency is determined by the spring constant and mass ofthe mechanical system and also by the elasticity coefficient of the gas.In the case of refrigeration, the elasticity coefficient of the gasincreases with both evaporating and condensing pressures. Consequentlythe natural frequency also increases. Therefore for best operation thefrequency of the electrical system powering the compressor needs to varyto match the mechanical system frequency as it varies with operatingconditions.

Methods of synchronising the electrical voltage applied to thecompressor motor windings with the mechanical system frequency are wellknown. For a permanent magnet motor used in a free piston compressor, aback electromotive force (back EMF) is induced in the motor windingsproportional to the piston velocity as shown in FIG. 8a. The equivalentcircuit of the motor is shown in FIG. 8b. An alternating voltage (V) isapplied in synchronism with the alternating EMF (αv) in order to powerthe compressor. U.S. Pat. No. 4,320,448 (Okuda et al.) discloses amethod whereby the timing of the applied voltage is determined bydetecting the zero crossings of the motor back EMF. The application ofvoltage to the motor winding is controlled such that the current iszero, at the time at which the EMF intersects with the zero level toallow back EMF zero crossing detection.

Various methods have been used to limit oscillation amplitude includingsecondary gas spring, piston position detection, piston positioncalculation based on current and applied voltage (U.S. Pat. No.5,496,153) measuring ambient and/or evaporating temperature (U.S. Pat.No. 4,179,899, U.S. Pat. No. 4,283,920). Each of these methods requiresthe cost of additional sensors or has some performance limitation.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a compact linearmotor which goes some way to overcoming the abovementioned disadvantagesor which will at least provide the public with a useful choice.

Accordingly in a first aspect the present invention may be said toconsist in an electric linear motor for driving a reciprocating loadcomprising:

a stator having a magnetically permeable core with at least one air gapand means for producing a non constant magnetic flux in said stator andsaid at least one air gap;

an armature having a structure which supports at least one permanentmagnet of which at least a substantial portion is located in at leastone of said at least one air gap, such that the interaction of themagnetic field of said at least one permanent magnet and said nonconstant flux in said at least one air gap producing a force on saidarmature, said armature in use being connected to said load and therebyreciprocating with respect to said stator; and

energisation means for controlling said means for producing analternating flux such that at least one end of said at least onepermanent magnet passes outside the region of substantially uniform fluxdensity present within said at least one of said at least one air gapduring a portion of the reciprocal motion of said armature.

In a second aspect the present invention may be said to consist in arefrigerator which uses a compressor characterised in that thecompressor and compressor motor are linear devices and said motorcomprises:

a stator having a magnetically permeable core with at least one air gapand means for producing a non constant magnetic flux in said stator andsaid at least one air gap;

an armature having a structure which supports at least one permanentmagnet of which at least a substantial portion is located in at leastone of said at least one air gap, such that the interaction of themagnetic field of said at least one permanent magnet and said nonconstant flux in said at least one air gap producing a force on saidarmature, said armature in use being connected to said load and therebyreciprocating with respect to said stator; and

energisation means for controlling said means for producing analternating flux such that at least one end of said at least onepermanent magnet passes outside the region of substantially uniform fluxdensity present within said at least one of said at least one air gapduring a portion of the reciprocal motion of said armature.

In a third aspect the present invention may be said to consist in avapour compressor comprising:

a piston,

a cylinder,

said piston being reciprocable within said cylinder, the vibratingsystem of piston, spring and the pressure of said vapour having anatural frequency which varies with vapour pressure,

a linear brushless DC motor drivably coupled to said piston having atleast one winding,

a DC power supply,

commutation means for electronically commutating said at least onewinding from said DC supply to provide a supply of current to said atleast one winding to reciprocate said piston,

resonant driving means which initiate commutation of said at least onewinding to thereby drive said piston at the resonant frequency of saidvibrating system,

current controlling means which determine the amount of said supply ofcurrent supplied by said commutation means, said determined amount ofcurrent being related to said resonant frequency, and which initiatecommutation of said at least one winding to thereby limit the amplitudeof reciprocation of said piston.

In a forth aspect the present invention may be said to consist in amethod for driving and controlling the amplitude of the piston in a freepiston vapour compressor wherein said piston reciprocates in a cylinderand wherein the vibrating system of piston, spring and the pressure ofsaid vapour has a resonant frequency which varies with vapour pressure,said method using a linear brushless DC motor having at least onewinding and comprising the steps of:

electronically commutating said at least one winding from a DC supply toreciprocate said piston, with commutations timed to drive said piston atthe resonant frequency of said vibrating system, limiting the amount ofcurrent in said at least one winding by limiting the value of aparameter which determines current supply during commutation to a valuewhich is a function of said resonant frequency.

The “evaporating temperature of the vapour entering the compressor” isalso referred to in this specification as the “evaporator temperature”.Likewise the “resonant frequency” is also referred to as the “naturalfrequency”.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-section of a linear compressor according to thepresent invention;

FIG. 2 is a cross-section of the double coil linear motor of the presentinvention in isolation;

FIG. 3 is a cross-section of a single coil linear motor;

FIG. 4 is a comparison between a single window prior art linear motorand a short stator linear motor according to the present invention;

FIG. 5 is an illustration of the flux lines due to the coil current in asingle coil linear motor of the present invention;

FIG. 6 is a graph of the motor constant versus magnet position for thepreferred embodiment of the present invention;

FIG. 7 is a cross-section of a single coil linear motor with partiallyangled pole faces;

FIG. 8a shows motor piston displacement and back EMF waveforms for afree piston compressor motor;

FIG. 8b shows an equivalent circuit for such a motor;

FIG. 9 shows an inverter for electronically commutating a single phasefree piston motor;

FIG. 10 shows graphs of maximum motor current as a function of frequencyand evaporation temperature for a motor of the present invention;

FIG. 11 is a block diagram of the motor control circuit;

FIG. 12 is a graph of RMS motor current versus motor winding currentdecay time;

FIG. 13 is a flow chart of the motor control timing program;

FIG. 14 is a flow chart of commutation time determination usingevaporator temperature and stroke time data; and

FIG. 15 shows motor piston displacement and motor current waveforms.

MODE(S) FOR CARRYING OUT THE INVENTION

The present invention provides a method for controlling a linear motorwith a number of improvements over the prior art. Firstly it has areduced size compared to the conventional linear motor of the typedescribed in U.S. Pat. No. 4,602,174 and thus reduces the cost. Thischange keeps the efficiency high at low to medium power output at theexpense of slightly reduced efficiency at high power output. This is anacceptable compromise for a compressor in a household refrigerator whichruns at low to medium power output most of the time and at high poweroutput less than 20% of the time (this occurs during periods of frequentloading and unloading of the refrigerator contents or on very hot days).Secondly it uses a control strategy which allows optimally efficientoperation, while negating the need for external sensors, which alsoreduces size and cost.

While in the following description the present invention is described inrelation to a cylindrical linear motor it will be appreciated that thismethod is equally applicable to linear motors in general and inparticular also to flat linear motors. One skilled in the art willrequire no special effort to apply the control strategy herein describedto any form of linear motor. It will also be appreciated that thepresent invention will be applicable in any form of compressor. While itis described in relation to a free piston compressor it could equally beused in a diaphragm compressor.

A practical embodiment of the invention, shown in FIG. 1, involves apermanent magnet linear motor connected to a reciprocating free pistoncompressor. The cylinder 9 is supported by a cylinder spring 14 withinthe compressor shell 30. The piston 11 is supported radially by thebearing formed by the cylinder bore plus its spring 13 via the springmount 25.

The reciprocating movement of piston 11 within cylinder 9 draws gas inthrough a suction tube 12 through a suction port 26 through a suctionmuffler 20 and through a suction valve port 24 in a valve plate 21 intoa compression space 28. The compressed gas then leaves through adischarge valve port 23, is silenced in a discharge muffler 19, andexits through a discharge tube 18.

The compressor motor comprises a two part stator 5,6 and an armature 22.The force which generates the reciprocating movement of the piston 11comes from the interaction of two annular radially magnetised permanentmagnets 3,4 in the armature 22 (attached to the piston 11 by a flange7), and the magnetic field in an air gap 33 (induced by the stator 6 andcoils 1,2).

A two coil embodiment of present invention, shown in FIG. 1 and inisolation in FIG. 2, has a current flowing in coil 1, which creates aflux that flows axially along the inside of the stator 6, radiallyoutward through the end stator tooth 32, across the air gap 33, thenenters the back iron 5. Then it flows axially for a short distance 27before flowing radially inwards across the air gap 33 and back into thecentre tooth 34 of the stator 6. The second coil 2 creates a flux whichflows radially in through the centre tooth 34 across the air gap axiallyfor a short distance 29, and outwards through the air gap 33 into theend tooth 35. The flux crossing the air gap 33 from tooth 32 induces anaxial force on the radially magnetised magnets 3,4 provided that themagnetisation of the magnet 3 is of the opposite polarity to the othermagnet 4. It will be appreciated that instead of the back iron 5 itwould be equally possible to have another set of coils on the oppositesides of the magnets.

An oscillating current in coils 1 and 2, not necessarily sinusoidal,creates an oscillating force on the magnets 3,4 that will give themagnets and stator substantial relative movement provided theoscillation frequency is close to the natural frequency of themechanical system. This natural frequency is determined by the stiffnessof the springs 13, 14 and mass of the cylinder 9 and stator 6. Theoscillating force on the magnets 3,4 creates a reaction force on thestator parts. Thus the stator 6 must be rigidly attached to the cylinder9 by adhesive, shrink fit or clamp etc. The back iron is clamped orbonded to the stator mount 17. The stator mount 17 is rigidly connectedto the cylinder 9.

In a single coil embodiment of the present invention, shown in FIG. 3,current in coil 109, creates a flux that flows axially along the insideof the inside stator 110, radially outward through one tooth 111, acrossthe magnet gap 112, then enters the back iron 115. Then it flows axiallyfor a short distance before flowing radially inwards across the magnetgap 112 and back into the outer tooth 116. In this motor the entiremagnet 122 has the same polarity in its radial magnetisation.

In the preferred embodiment of the present invention the length of thearmature (tooth) faces only extends to, for example, 67% of the maximumstroke (where the edge of the magnet extends to at maximum power output)of the magnet. This is seen in FIG. 4 where a conventional prior artlinear motor is visually compared against the present invention variableconstant design of equivalent power output, both at maximum stroke. Itcan be seen that the outer edge 200 of the stator tooth does not extendas far as the outer end of the magnet 201. Similarly the inner edge 203of the other stator tooth does not extend to the inner end of the magnet204. In contrast in the prior art design the edge of the magnet 205 doesmatch up with the edges of the stator teeth 206,207 at maximum stroke.

At strokes less than, for example, 60% in the present invention themagnet 70 will be in an area of uniform flux density as indicated by theregion “a” to “b” in FIG. 5, which roughly corresponds where the statorteeth 71 extend to. As the stroke increases past 60% the flux densityencountered by the magnet edge 70 reduces as it enters the fringeportion (non-uniform flux density) of the air gap magnetic field—thearea outside “b” in FIG. 5.

In a further embodiment shown in FIG. 7, a stator for a linear motor isshown with angled pole face 503. In its centre the pole face 503 has aflat section 500, which results in the air gap facing that sectionhaving substantially uniform flux density. The end of the pole face 503,is angled to give a more progressive transition from the uniform fluxdensity of the centre 500, to the fringe portion 502 (non-uniform fluxdensity) at the end of the pole face 503. Similar to the proceedingembodiments the armature magnet 504, would be driven outside the area ofuniform flux density 500, and into the fringe portion 502 of non-uniformflux density.

The “Motor Constant” is defined as the force (in Newtons) generated onthe magnet by one Ampere in the motor windings. The motor constantcurve, shown in FIG. 6 shows how the Motor Constant 300 for the presentinvention varies with magnet position. Equally the “Motor Constant” canbe defined as the back EMF (in Volts) generated when the magnet ismoving at one meter/second. When the magnet is in the fringe field(outside “b” in FIG. 5), because of the reduced magnetic coupling, morecurrent will be required to generate a given force when compared to thatin the uniform flux region (from “a” to “b” in FIG. 5). This results inthe “variable” motor constant curve 300 associated with the presentinvention short stator linear motor as shown in FIG. 6. This contrastswith the “constant” motor constant curve 301, also seen in FIG. 6,inherent in the conventional prior art linear motors.

With the motor constant curve 300 shown in FIG. 6 at low and mediumstrokes (corresponding to strokes of −3 mm to +3 mm) it will be apparentthe present invention has a high motor constant relative to anequivalent convention motor 301, (with less turns and a greater volumeof core material). A higher motor constant corresponds to more efficientoperation (due to lower invertor losses), therefore at lower poweroutput the present invention is more efficient than an equivalentconventional prior art linear motor. It also reduces the required crosssectional area of the core.

At high strokes the motor constant is low at the times when the currentis increasing most rapidly. This makes it possible to get more currentinto the motor and thus extract more power from the motor at maximumstrokes as compared to an equivalent conventional prior art linearmotor. Also such a design with a variable constant that is lowest atmaximum stroke tends to make motors driven by square wave voltages moreefficient.

Control Strategy

Experiments have established that a free piston compressor is mostefficient when driven at the natural frequency of the compressorpiston-spring system. However as well as any deliberately provided metalspring, there is an inherent gases spring, the effective spring constantof which, in the case of a refrigeration compressor, varies as eitherevaporator or condenser pressure varies. The electronically commutatedpermanent magnet motor already described, is controlled using techniquesincluding those derived from the applicant's experience inelectronically commutated permanent magnet motors as disclosed in U.S.Pat. No. 4,857,814 and WO 98/35428 for example, the contents of whichare incorporated herein by reference. Those references disclose thecontrol of a 3 phase rotating motor, but the same control principles canbe applied to linear motors. A suitable linear motor need only be asingle phase device and a suitable inverter bridge circuit for poweringa motor can be of the simple form shown in FIG. 9.

By monitoring back EMF zero crossings in the motor winding currentcommutation can be determined to follow the natural frequency of thepiston. Since there is only a single winding, the current flowingthrough either upper or lower inverter switching devices 411 or 412 mustbe interrupted so that back EMF can be measured. Controlling the currentthrough the motor winding in accordance with detected back EMF ensurescurrent and back EMF are maintained in phase for maximum systemefficiency.

The frequency of operation of the motor is effectively continuouslymonitored as frequency is twice the reciprocal of the time between backEMF zero crossings. Furthermore according to WO 98/35428 the currentdecay time through free wheel diodes 413 and 414 after commutation hasceased is directly proportional to the motor current and thus a measureof motor current is available.

The maximum motor current that can be employed before the pistoncollides with the cylinder head of the compressor varies depending uponthe evaporator temperature and the natural frequency of the vibratingsystem

FIG. 10 shows graphs of maximum permitted motor current against naturalmechanical system frequency and condenser temperatures for differentevaporating temperatures. These show the dependence of maximum motorcurrent on both these variables. They also demonstrate that condensertemperatures are proportional to mechanical system frequency and thusmaximum current control can be achieved without the need for measurementof the third variable, condenser temperature.

The motor control circuit according to this invention is shown in FIG.11. It utilises the observation that mechanical system frequency isrelated to condenser temperature. In this invention the back EMF signalinduced in the motor windings 1 is sensed and digitised by circuit 402and applied to the input of a microcomputer 403 which computes theappropriate timing for the commutation of current to the motor windingsto ensure that the current is in phase with the back EMF. Thesecommutation timing signals switch an inverter 404 (as shown in FIG. 11)which delivers current to the motor windings 401. The microcomputer 403also measures the time between back EMF zero crossings and thereby theperiod of the EMF waveform. The natural oscillation frequency of themechanical system is the inverse of the period of the EMF waveform. Themicrocomputer 403 therefore has a measure of this frequency at alltimes.

The conventional temperature sensor 405 for measuring the evaporatortemperature for defrost purposes is utilised and its output is digitisedand supplied as a further input to microcomputer 403.

According to the present invention one method of limiting maximum motorcurrent and thus maximum displacement of the piston is for themicrocomputer 403 to calculate a maximum current amplitude for each halfcycle of piston oscillation and limit the actual current amplitude toless than the maximum. WO 98/35428 discloses a method of measuring motorcurrent in an electronically commutated permanent magnet motor byutilising the digitised back EMF signal in an unpowered winding tomeasure the time taken for the current in the motor winding to decay tozero. Use of this technique in the present invention enablesmicrocomputer 403 to limit maximum power without the need for dedicatedcurrent sensing or limiting circuitry. The RMS motor current is directlyproportional to the time duration of current decay through the“freewheeling” diodes 413 or 414 after the associated inverter switchingdevice has switched off. The current decay results of course from themotor winding being an inductor which has stored energy duringcommutation and which must be dissipated after commutation has ceased. Agraph of RMS motor current against current decay duration (which is asimplification of FIG. 6 in WO 98/35428) is shown in FIG. 12.

Another preferred method is to limit the time that the current iscommutated on instead of limiting the maximum current value. FIG. 15shows the current waveform under such control. This is in effect pulsewidth modulation (PWM) with only one modulated current pulse percommutation interval. With this method a delay time from the back EMFzero crossing is computed to minimise the phase angle between the MotorCurrent and the back EMF for maximum efficiency. The invertor switchsupplying current is turned off at a time in the motor half cycle toallow, after a current decay period, time to monitor zero crossing ofthe back EMF to determine the start commutation for the next half cycle.The commutation time is also compared with a maximum commutation timeappropriate to the motor frequency and evaporator temperature to ensuremaximum amplitude of the piston stroke is not exceeded.

A flow diagram of the microcomputer control strategy to implement thismethod is shown in FIGS. 13 and 14. Referring to FIG. 13 when thecompressor is first powered (421), or is powered after sufficient timedelay to ensure pressures are equalised in the refrigeration system, thecompressor runs at a minimum frequency. The stroke period of thisminimum frequency is measured as Run_Stroke and shown in themicrocomputer as Low_Stroke and a minimum Commutation Time is set forthis value (428). For each subsequent stroke the stroke period ismeasured and defined as the parameter Run_Stroke (424). The differencebetween Run_Stroke and Low_Stroke is computed (431, FIG. 14). Thisdifference is called Period_Index. The Period_Index is used in thissub-routine as an index pointer in a lookup table of maximum commutationtimes for different stroke times (frequencies). This table is called thePulse_Limit_Value Table. In this instance there are 7 lookup tables (433to 439) corresponding to 7 ranges of Evaporating Temperature (440 to465).

The motor control circuit is typically included in a Temperature Controlloop in the conventional manner in order to maintain the temperature ofthe enclosed refrigerated space of the refrigeration system. Thiscontrol loop will be setting desired values for the power to be appliedto the motor windings depending on the operating conditions of therefrigeration system. These values of desired power will correspond tovalues of commutation time. These values of Commutation Time arecompared on a stroke by stroke basis with the Pulse_Limit_Value (440,FIG. 14). If the Desired value of commutation time is greater than thePulse_Limit_Value then the commutation time is limited to thePulse_Limit_Value. This value sets the Commutation Timer (425) whichcontrols the ON period of the relevant inverter switching device. Aspreviously explained, Motor current can also be used in a similar mannerto limit power applied to the motor to safe levels, but even wherecommutation time is being controlled it is desirable to measure motorcurrent in the manner previously described and compare it with a storedabsolute maximum value (426) which if exceeded will cause themicrocomputer program to reset (427).

Of course other methods of determining maximum commutation time and/ormaximum current value are feasible, for instance if the microcomputer issufficiently powerful, for example recent advances in DSP chiptechnology, these values can be computed directly without the need forlookup tables.

If the DC power supply Voltage supplied to the inverter bridge of FIG. 9varies significantly this will result in variation of Motor Current forany given commutation time which should be allowed for. It may bedesirable for maximum accuracy for the microprocessor to sense this andcompensate accordingly.

It will be appreciated that use of the present invention in arefrigerator reduces the profile, size and weight of the motor comparedto that of conventional designs. Also because the mass of the movingparts is lower than that of a conventional refrigerator compressor:

the level of vibration is reduced,

the noise level is reduced,

the working stresses on the moving parts are reduced.

What is claimed is:
 1. An electric linear motor for driving areciprocating load comprising: a stator having a magnetically permeablecore with at least one air gap and means for producing a non constantmagnetic flux in said stator and said at least one air gap; an armaturehaving a structure which supports at least one permeable magnet of whichat least a substantial portion is located in at least one of said atleast one air gap, such that the interaction of the magnetic field ofsaid at least one permanent magnet and said non constant flux in said atleast one air gap producing a force on said armature, said armature inuse being connected to said load and thereby reciprocating with respectto said stator; and energisation means for controlling said means forproducing an alternating flux such that at least one end of said atleast one permanent magnet passes outside the region of substantiallyuniform flux density present within said at least one of said at leastone air gap during a portion of the reciprocal motion of said armature.2. An electric linear motor as claimed in claim 1 wherein said means forproducing an alternating magnetic flux comprises at least one coil woundaround a portion of said stator and energised with a non constantvoltage.
 3. An electric linear motor as claimed in claim 2 wherein saidenergisation means comprises a commutation circuit including a directcurrent power supply, switching devices connected to said power supplycurrent to said at least one coil and a programmed digital processorincluding memory and input-output ports, at least one of said portsbeing connected to said commutation circuit to supply switching controlsignals thereto.
 4. An electric linear motor as claimed in claim 1wherein the displacement of said at least one permanent magnet at whichsaid at least one end of said at least one magnet passes outside saidregion of substantially uniform flux density is 67% of the maximumdisplacement.
 5. A refrigerator which uses a compressor characterized inthat the compressor and compressor motor are linear devices and saidmotor comprises: a stator having a magnetically permeable core with atleast one air gap and means for producing a non constant magnetic fluxin said stator and said at least one air gap; an armature having astructure which supports at least one permanent magnet of which at leasta substantial portion is located in at least one of said at least oneair gap, such that the interaction of the magnetic field of said atleast one permanent magnet and said non constant flux in said at leastone air gap producing a force on said armature, said armature in usebeing connected to said load and thereby reciprocating with respect tosaid stator; and energisation means for controlling said means forproducing an alternating flux such that at least one end of said atleast one permanent magnet passes outside the region of substantiallyuniform flux density present within said at least one of said at leastone air gap during a portion of the reciprocal motion of said armature.